The disclosure relates to a switched-mode power supply device, the switching frequency of which varies with load, and particularly to a technique of reducing switching loss of the switched-mode power supply device under light load.
A power factor improvement circuit described in U.S. Pat. No. 6,448,745 (Patent Document 1) is known as a switched-mode power supply device of the prior art. This power factor improvement circuit is intended to improve a power factor when running in a discontinuous mode, and includes a controller which inputs an input voltage and an output voltage.
Based on the input voltage, the output voltage, and an ON time of a switching element, the power factor improvement circuit calculates an OFF time during which a current through a reactor will decrease from its peak to zero and performs ON-OFF control of the switching element using ON and OFF signals of the ON time and the OFF time.